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How much thrust is lost through a curved exhaust nozzle?
In modern VTOL aircraft energy is usually directed through nozzles to achieve vertical lift. How much energy is lost when changing the direction of flow in static thrust conditions?

Reminds me of that Mythbusters episode where they bent the barrel of a rifle 90 degrees and it was still lethal.  One of my favorites!

Note that the Rolls-Royce Pegasus always forces the exhaust around a bend, no matter which direction the nozzle points...

My naive thought would be that the direction of the vectoring force (the "bending" force) is in this case upwards (remember those rotating lawn sprinklers using some of the water pressure to propel their rotation) which means that not too much power would be lost to the vectoring, only rather small amount to increased friction/compression in the bend.

For the F135 engine in this photo, thrust in hover is only about one per cent less than maximum thrust, if Pratt & Whitney's data sheet is to be believed.

Maximum Thrust Class 41,000 lbs
...
Hover Thrust 40,650 lbs

The Hawker Harrier's maximum thrust was about 20,280 pounds.
An approximation of its maximum vertical thrust at low airspeed is given by its maximum vertical landing weight, 19,918 pounds, again just about one per cent less.

Thanks for the info and the links! On the F135 it lists the lift fan as part of the hover thrust so it's hard to know for sure as the engine splits it's maximum 27000 lb thrust (military power). 18,680 lbs of thrust comes directly from the rear nozzle. Something like 8320 lbs worth of thrust is sacrificed to drive the more efficient lift fan which then provides once again 18,680 lbs.

The trick seems to be to constrict the flow at the nozzle enough that the effects of the 90 degree bend on the flow are minimized.  In effect it is like blowing up a crooked balloon and comparing its "thrust" to a perfectly round one.  The flow at the bend is much slower than at the nozzle.

"a few percent"? Based on those numbers, the loss is 0.86% for the F135 , so "a fraction of a percent" would be more accurate. (It's a 1.79% loss for the Harrier, so I'll cut you slack on that one.) :D

Damn I sure love the F135 engine.

A 90 degree bend in a pipe where the radius of the bend is of order ~one pipe diameter creates the same pressure drop as a length of that same pipe of order ~ten to fifteen times the pipe diameter.

Which is how much, compared to an unmodified pipe?  Five per cent?  Fifty per cent?

do not know in terms of percent.

There is very little duct loss due to the double 90 degree bend in the exhaust. Hawker chief designer Sidney Camm used a similar design in the Hawker Seahawk a 1950s Naval fighter, which had twin exhausts exiting behind the trailing edge wing root. Camm found this arrangement had lower duct losses than a long duct exiting under the tail. This was an important consideration given the limited power available from early jet engines. Camm used the same duct arrangement on the Hawker Kestrel / Harrier, working closely with Stanley Hooker, chief engineer from Bristol engines (later merged with Rolls-Royce) who developed the Pegasus.

Hi! Do you have any public sources you could like for those that would like to know more?

Hi, take a look at Stanley Hookers’ brilliant autobiography called ‘Not Much of an Enginneer’. He writes about the design of the Pegasus engine as well as working with Sidney Camm.

That book is online.  Which pages mention the Hawker Seahawk ducting?


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Can a different company manufacture an FAA approved airframe or engine?
If a certain engine architecture or airframe design was granted FAA certification and then
licensed to, manufactured and then sold by another company, would it still be certified?
If not, would this change the certification process at all?

While I don't know anything about the licencing or certification issues involved, there are several companies building versions of the Piper Cub.

Also, Cessna licensed European production of several models - the 172 & 150, perhaps others, to a French company, Reims Aircraft.

If the question is about manufacture of a complete aircraft or engine under license it would not affect the design approval under the Type Certificate in a significant way. The TC is the reference design that was approved and it hasn't changed. There may be a top level change (an STC or Amended TC) that changes the designation so it is tied to the actual manufacturer and location. This is done to simplify configuration management.
The bigger impact is that manufacturing approval of the a/c or engine to the TC requires a Production Certificate (if you don't have a PC, the Cert Authority has to approve each a/c or engine produced.) PCs are location specific.
So when Boeing moved 787 production to South Carolina, Boeing had to get a new PC even though the TC didn't change.
Airbus A320s built in Mobile Alabama are built under a PC granted to Airbus Mobile.

Yes, the FAA has a process for this called PMA - Parts Manufacturer Approval.
In summary:

(PMA) Is a combined design and production approval for modification and replacement articles. It allows a manufacturer to produce and sell these articles for installation on type certificated products. Federal Aviation Administration Orders 8110.42 and 8120.22 prescribe the approval procedures for FAA personnel and guides applicants in the approval process.

The holder of the Type Certificate must grant PMA authority to another manufacturer, and have it approved by the FAA.
There is more information available through the FAA website here:  FAA PMA

The OEM doesn't grant PMA authority per se; it issues a PMA support letter which speeds things up for the company seeking the authority from the FAA, but this doesn't necessarily have to happen.  There are oodles of PMA parts out there that were approved without the support , or even knowledge, of the OEM.  The PMA applicant only has to demonstrate that their part is effectively a copy of the OEM part from a structural and performance perspective.  However, if the part is subject to an AD, the FAA will generally refuse PMA authority, and if the part is already in production, may revoke it.

@JohnK, thanks for the feedback.  I didn't want to go into too much detail for a couple reasons:  1.  My knowledge is limited, and...  2.  Someone really needing to know this stuff ought to go to the source.  Do you have any wording suggestions to make this a better answer?  Finally, regarding "oodles of parts" without knowledge or consent of OEM, that may be true for simple parts where the designer has surrendered intellectual property rights, but for more complex (especially electronic, which is my background...) design data is a valuable commodity (continued...)

... a commodity not readily surrendered or reverse engineered without violating supply contracts, patents, or other legal "ownership" things.

Agreed on that point.  The FAA won't approve a PMA on a complex component or major assembly without OEM support.  Where the boundary is, is kind of nebulous.

The PMA is not a design approval. It is manufacturing approval that says you can manufacture a part to the approved design of a Type Certificate or STC. Every supplier to an OEM normally gets a PMA for every part they supply to the OEM. The big exception is TSO'd parts. A TSO is a design and manufacturing approval given to the that supplier, but it still needs to incorporated into the TC or STC of an aircraft. If the question is about building the complete aircraft or engine (engines have their own TC), there are different rules as the a/c or engine is not a part.

@Gerry, if my answer leads you to believe that I think, or am stating, that PMA is a design approval then please explain how I can clarify that it is not.  Also, I don't necessarily agree that every supplier "normally" gets PMA.  PMA would allow a sub-tier supplier to sell and ship parts directly to third party maintenance providers, and from my experience most aircraft TC holders like to control this.

I was misled by the quote you provided from the FAA. Further reading of their summary is   subject to interpretation. They refer to PMA Design approval in the case of a new manufacturer showing their part is identical to the original part. The original part's design approval would be the TC or STC, which is still the design basis for the new part. This is the area where OEM support is a factor. With it, you can do conformity inspection to the OEM documentation. Without it, you're showing how you reverse engineered/duplicated an existing part.

As to suppliers getting PMAs, the last two companies I worked for were both suppliers to Boeing and Airbus. We had PMAs for our parts. That allowed our QA to sign the 8130-3  prior to shipping. We built parts for production and for spares. I believe there were contractual restrictions regarding sales of the spares. I know we shipped direct to airlines, but the order may have come through the OEM.

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Can you augment turbofan thrust using steam injected into the bypass?
Would using waste heat from the core exhaust to turn water into steam increase takeoff thrust when injected into the bypass before/aft of the fan?
If so, which would be better?
Could this possibly increase the effective pressure ratio or flow velocity of the air aft of the fan?

Can you explain in more detail where the "waste heat" would come from and how exactly you think it could be extracted? Put a massive heat exchanger right behind the engine? Also, why water? When it comes to volume of gas produced per amount of heat, water is one of the worst possible choices.

I don't think there would be a benefit to water injection in the fan stream.
Water injection in the core stream can be beneficial.  When the water converts to steam in the combustor, it absorbs a ton of energy and lowers the turbine entry temperature.  This allows you to burn more fuel and add more energy.
The bypass flow is not up against any temperature limit where this tradeoff would make sense.

Thanks for your response, however I don't think you understood the question. I am not suggesting water injection in the bypass, I'm suggesting using the excess heat from the core to heat water turning it into steam (expanding it) before injecting it. This could possibly increase the effective pressure ratio of the fan. I since updated the question to reflect this.

@usernamechecksout What excess heat? There's no core cooling system or anything like that in a typical turbofan, as that would be counter-productive. Any heat that you remove from the core downstream of the combustion chamber just decreases the energy available for the turbine.

@TooTea Using the heat from the exhaust which is usually around 500-600C. This heat energy is mostly wasted.

What you describe is basically an anti-afterburner. Cooling the exhaust would slow down the velocity reducing thrust. You might recover some of that with the increased mass from the steam and being a small molecule H2O can offer slightly better specific impulse but the enthalpy of vaporization is huge.
As a counter-example the heat exchanger of the P51 slightly added thrust because it heated the air passing through it.

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How do rate gyros and other sensors send information to the flight control computer in an F-16?
How do rate gyros and other sensors send information to the flight control computer in an F-16? I mean is it a hardwired communication or it is done through a bus? If by bus, what is the bus called?
I have read some articles about A-Mux. Is it the bus that the gyros transmit data through? I have the same question about the actuators. How does the flight control computer send information to the actuators? By bus or hard-wired? If you have references, that will be appreciated.

Modern rate gyros often have digital interfaces. The F-16 has had both analog and digital flight controls and the interfaces are likely to be significantly different within each type.

I've never heard of A-Mux in respect to aviation. Any links to relevant articles?

Please refer to this link: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830004825.pdf

The F-16 uses a MIL-STD-1553 data bus and was the first aircraft to do so.
References:

https://www.testandmeasurementtips.com/making-measurements-mil-std-1553-connections/
https://www.aim-online.com/wp-content/uploads/2019/01/aim-ovw1553-u.pdf

Does any version use dedicated ports in flight control computer for the sensors? For example look at the following reference page 32, figure 20 and the paragraph above it:https://apps.dtic.mil/dtic/tr/fulltext/u2/a128624.pdf


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How many actuators does the F-35 have for each primary control surface?
How many actuators does the F-35 have for each primary control surface like rudder, stabilator or aileron? And is it active/active or active/standby?

Just one per control surface.  Though the airplane is unique with using a single Electro-Hydrostatic Actuator (EHA) per control surface as opposed to a centralized hydraulics system.
https://www.f35.com/in-depth/detail/up-in-the-air-understanding-the-f-35-air-vehicle
I don’t know what you mean by active/standby but the surface actuators are commanded by inputs from the flight control computers, which respond as needed to stick and rudder inputs from the pilot to achieve a desired aircraft reaction to a control input.

When there are two actuators per surface, active-active means that both are commanded and energized; if one malfunctions, you may have force fight. Active-standby means that only one is active at a time; if one malfunctions, the other one takes over while the first one is de-energized. The downside is there is a time lag between the takeover and your surface may run away during that time.


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What does pre/post modification mean in Airbus MPD?
What is the difference between POST MOD (with partial mod i.e SXXXX) and PRE MOD (with partial mod) in the applicability column of an Airbus MPD (Maintenance Planning Document)?

It means that operator have to check what status of the particular MOD have the subject A/C, pre or post. Therefore it affects operators MP.

"Post Mod" means the modification issued by the manufacturer was carried out on an aircraft, and "Pre Modification" has the opposite meaning.


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Can the overwing exit slide be used when ditching in an A320?
I want to ask if an overwing exit slide in an Airbus A320 can be opened at all when landing or ditching. Is the exit still usable? Or it is blocked and we have to move the passenger to another one?
And in ditching is the water level one of the reasons why it will not be opened and the reason the exit is blocked? If it was the case, why to consider the primary exit in ditching is the over wing exit? If it is deployed we will not use it in ditching.

It's always possible that a door becomes blocked during a landing and that other doors must be used instead. That said, your title and question are a bit confusing and somewhat contradictory. The "Miracle on the Hudson" clearly shows passengers using the wing exits after a ditching, however they did not use the exit slide. Which are you asking about? Also, rising water levels (from holes in the fuselage and from someone opening a rear door) did cause the rear exits to be unusable. A little more clarity will help give a better answer.

Related: Why did the US Airways Flight 1549 passengers stay on the wings?

Let me clarify , our trainer said that in ditching or landing if the overwing slide did't deployed ( if it did not came up from the fuselage at all ) then the exit must be blocked and we have to direct passengers to another exit , and I am not convinced of that , that is the case I am asking about ... is it blocked in this case or not ?  ... I saw many videos about ditching and the water level is covering the fuselage where the slide is stored and maybe that is why it doesn't open ... if the water covering the container then It can't be opened why did she said that we have to block the exit?

And if it is the case of water level preventing the deployment of the slide then we should use the exit not blocking it , right ? Because in ditching we don't use the slide and the primary exits are the over wing' exits , right ?

The over overwing exits are secondary exits in a ditching and they should be used only if it becomes absolutely necessary to use them. The reason is because, the escape slides of the overwing exits are just slides and not rafts as they cannot be used as long term flotation devices. Moreover, unlike the slide rafts, the escape slides on the overwing exits cannot be detached from the aircraft. If the aircraft sinks, it takes the slides with it. The rafts can be separated away from the aircraft and can be used as small dinghy to hold the passengers. Furthermore, the rafts also have a loop to which you can attach the survival kit.

Here you can see that the A320 has four slide rafts (fwd and aft doors) and two overwing escape slides.
If the usage of overwing exits become necessary then they can be used in a ditching. In the Hudson landing, they used the overwing exits because the aft exits were rendered useless due to the considerable amount of impact damage received by the aft fuselage of the aircraft. Nonetheless, there are ditching related safety devices in the overwing exit. This is because in a ditching, the wing may get slippery. So, there are life lines that can be attached to assist passengers to escape the aircraft over the wing. This is what that yellow hook you see on the A320 wing is used for. So, yes wing exits can be used in a ditching, but they remain as secondary exits as long as all the other four exits are in working order.

Why did they make it so that the overwing slides can't be used as rafts?

@Sean Sounds like a separate question, but perhaps it's just because you can't fit all those extra features into the limited space available. An overwing exit is pretty much just an oversized window, there's much less room for the slide than in the main passenger doors.

@TooTea The slide is contained in the fuselage or wing-body fairing, not in the door, but yes, even there the space is limited. Also, the attach point of the slide is likely to be at the waterline, maybe even below, compared to a main door slide. See illustration.


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What options do the crew have when passengers go berzerk?
In one recent incident with an American Airlines flight from São Paulo (GRU) to New York (JFK or LGA, dunno), the crew decided to land in Brasilia (BSB), Brazil because one couple was fighting (source in Portuguese).
In this case, the crew decided to make a landing in an airport outside their flight plan, and remove the fighting passenger couple from the aircraft. Because of Brazilian regulations on the crew working hours, the aircraft was then grounded for the day.
In the scenario where:

passengers aboard an international flight (during flight) begin fighting between themselves (the shouting at each other, possibly bare-fist only kind of fight) and refuse to heed the flight crew's warnings and requests (but not in a life-threatening way), what other options could the crew have besides the one used in the incident above?

There is a 1. in the question, but not a 2.?

What options do the passengers have when the crew go berserk?

Cut the oxygen to the cabin by turning off the compressors, heh, heh.

@TomMcW: That could be a good question as, if we exclude hijacking, the statistics for serious harmful acts are against pilots.

@chirlu corrected. thanks.

@mins yeah, the pilot's burden is a great one. It would compare to the responsibility of a surgeon, he got lives at stake in a very delicate environment.

@mins "voluntary accidents" I like that phrase. Unfortunately with cockpit doors armored like a Brinks truck the people in the cabin are pretty much SOL. You've gotta trust your pilots. I suppose, though, that is the same with or without hardened cockpits. If one of the guys with the controls wants to crash a plane even the other pilot would be hard-pressed to stop it. Y'know, that might make a good question after all.

My flight attendant friend told me they are taught to ram them with the cart

Re "possibly bare-fist only kind of fight" -- what, you mean as opposed to the possibility that they might have planned ahead and brought boxing gloves?

See Chapter 4 of this IATA document for flight crew management of unruly passengers.
http://www.iata.org/whatwedo/safety/Documents/Guidance-On-Unruly-Passenger-Prevention-And-Management-1st-Edition.pdf
In general, they will weed a lot of the negative behavior out on the ground prior to boarding by looking for signs of agitation or aggressiveness towards other passengers or crew.  Should a passenger becom unruly while enroute, the following is generally done.
Assess the situation and level threat:  what's happening? who is doing it? what are their grievances? Are the legitimate e.g. A family member having a medical emergency or trivial e.g. Passenger in 22B is drunk and aggressive?
If there are other than legitimate reasons for the disruption, action in taken using the use-of-force continuum:
Physical presence - flight crew will be nearby watching subject.
Verbal commands - confront subject.  Inform them of both the airline's and Law Enforcemnt's zero tolerance policy toward their exhibited behavior.  Inform them if they continue with this behavior the aircraft will be landed, they will be placed under arrest by LE on the ground, and that penalties for their actions are severe.  Try to calm them down as well, but do NOT serve them alcohol, even if requested.
If behavior continues, quietly inform the cockpit crew of the disruption and they will make arrangements to divert to nearby airport and inform local airport police and other LE agencies to be ready on the ground.
If passenger has become so unruly that they are posing an immediate threat to other passengers around them, physical force can be used to restrain them by the flight crew.  This poses a high risk and should only be done as a last resort to regain control of the cabin.  If there is a federal Sky Marshall(s) on board they can assist the crew here as well.
Restraints such as handcuffs and the waist belts may be used to immobilize subject until LE can take over.  Divert will become urgent.
Electro Muscular Disruptors - flight crews are now being equipped with and trained on the use of the TASER EMD systems to immobilize unruly passengers in order to restrain them.
If subject's actions pose an immediate threat to life and limb of passengers and threaten the safety of the aircraft and crew (VERY low probability but it is is a contingency), deadly force may be used to neutralize the threat.  Near as I know some Capt./FOs do carry firearms as do Sky Marshals.  I'm not aware of guns being issued to flight attendants.

Carlo, the kind of behaviour I am meaning in the question is the one described in pages 27 and 28 of the document linked by you. Since the scenario is in-flight, that would be the "cabin crew actions" part of the helpcard. But it is blanked out. Do you know what is in that sections?

Second to last paragraph... do you mean "now being" instead of "not being"?

In the incident pictured at the question, the quarrying couple was not arrested upon landing, but they were barred from re-boarding the plane (and probably lost their tickets).

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text:
Is the MTOW determined by the wheel maximum supported weight?
I was on a LATAM flight a couple days ago, and decided to have a read through their VAMOS magazine. As a lot of airlines do, they had a couple of pages on their fleet, and also a double spread on one specific plane.
This plane was the 777, which was a little bit surprising as they are about to get rid of those I think. Anyway, it had all the usual facts and figures, and also a couple of highlights about the plane.
I managed to find a web copy of the magazine, and the picture is below:

https://issuu.com/spafax/docs/vamos_latam_junio_2017_web
One of these highlights is the wheels, and it says (roughly translated):

Each of the wheels is capable of supporting a maximum of 29,294 kg.

My first reaction was "That's pretty impressive!", then "I wonder how much above the MTOW that is?" The MTOW is specified at the side there: 351,530 kg, which correspondes to the 777-300ER.
So 29,294 x 6 x 2 = 351,528
Oh. That's under the MTOW. I wonder if there's some rounding off here:
So 351,530 / 2 / 6 = 29,294.166 - yes there is.
I was surprised that the maximum each tyre/wheel could take summed up to the MTOW, as I thought that they would experience larger forces on landing. Also, what would happen if a tyre blew?
So, does the maximum load for a tyre/wheel have anything to do with the MTOW? Or is this just marketing guys jumping the gun?
I have seen this question How is Maximum Take Off Weight determined? and that says that the manufacturers determine it, and it is based on the required payload of the aircraft and the design configuration, which doesn't really answer this question.
Edit with extra info mentioned in comments:
When stationary, the wheels will have a total force on them of 351,530 kgs, which according to this magazine is their max. When landing, the wheel like have to bear a weight of 251,290 kgs, plus the force of the landing. In my mind, the extra force of the landing would push the total above the MTOW, but this is just in my mind so I'm probably wrong :P

Maximum take off weight is often higher than maximum landing weight specifically for that reason. There are quite a few aircraft that can take off but have to burn fuel before it can land again.

@Sanchises right. I get that. But according to this, when the plane is at MTOW and stationary, the wheels are at their max. So surely the impact of landing with 70% of that weight would exceed that max?

"surely"? have you run the formula? or is "surely I am right in thinking this and I can't be wrong?" :P
If you run the formula you'll see that one element is the weight, the other the speed of impact. You could land with 1% of the MTOW and still completely destroy your aircraft if the speed is excessive.

@Federico I'm asking you guys for the formula :P

ok, point taken :D (but be wary, one thing is the MTOW, another is the maximum landing weight)

Seat back brochures aren't an accurate representation of the engineering behind this. The wheels are not at the maximum loading at MTOW. The weight is more determined by stopping the aircraft prior to V1 during take off given the configuration of the aircraft. It isn't really about how much the tires hold. There is no one "formula" to cover this.

@RonBeyer so, to answer my question, Marketing People jumped the gun?

@CalvT븃 Not so much "jumped the gun", but trying to impress passengers with numbers that they probably don't fully understand. The gear system itself (tires included) can withstand landing pressures significantly higher than that.

@RonBeyer that's what I thought. Do you have any numbers on that?

14 CFR 25.301: Loads, 14 CFR 25.303: Factor of Safety, 14 CFR 25.473: Landing Load Conditions, 14 CFR 25.479: Level Landing Conditions, Basically anything in 25.471-25.519

Let me rephrase that: any numbers specific to the 777 which help answer this question?

I feel your initial question was whether the figures for the wheel were accurate in the brochure or significantly underestimated by the marketing guy doing a simplistic division MTOW/# of wheels (a good question imho), but the title was a bit remote... and now you are overwhelmed with details and almost asked to justify your question :-) Keep cool!

@mins I'm calm don't worry :) - my question in a nutshell for you - "are these figures correct, as they don't make sense to me, and if they aren't, what are the correct figures?"

I can't answer, and maybe it's difficult to find the information in the technical specs, but for sure there is a safety margin that may be 100% or more of the static load (the weight on the wheel, which depends on the position of the center of gravity). Effect of wind while on the ground (negative lift) must also be taken into account.

@CalvT븃, I suspect you are correct. They jumped the gun (assumed) that is max of each tire, because I find it hard to believe that the tires don't have some redundancy built in to them, so that if one blows while taxing, they all wouldn't then blow out because of the increased load.

@CalvT븃 I noticed that you divided the MTOW by 12 tires, but there are actually 14 tires. Unfortunately, the nose wheel tires appear to be different size, so it wouldn't be accurate to divide by 14, but since some of the weight is on the nose wheel, the total MTOW will not be on the main wheels.

English for picture description : https://www.latam.com/vamos/get-inspired/facts-to-learn-about-the-boeing-777

Boeing1[pdf] specs for 777-300ER:

Max taxi weight: 777,000 lbs (352,442 kg)
Max take off weight: 775,000 lbs (351,535 kg)
Max landing weight: 554,000 lbs (251,290 kg)

Bridgestone2 specs for approved tires:

Main: 66,500 lbs (30,264 kg) x 12 = 798,000 lbs (361,967 kg)
Nose: 44,500 lbs (20,185 kg) x 2 = 89,000 lbs (40,370 kg)
Total tire rated load = 887,000 lbs (402,337 kg)

So at taxi weight on all 3 gear there's roughly a 110,000 lbs / 50,000 kg margin (about 14%).
At max landing weight, main gear only, there's a 244,000 lbs / 110,677 kg margin (44%).
At MTOW there's about a 112,000 lbs / 51,000 kg margin (14%) on all gear, but only a 23,000 lbs / 10,400 kg (3%) margin after nose gear lift off.
But, when the nose gear lifts off the ground the wings are supporting nearly all of the weight. Their numbers are pretty close to what the manufacturers list. I think you're just not accounting for the nose gear at takeoff and the weight difference at landing.
As far as Is the MTOW determined by the wheel maximum supported weight? It's the other way around. The MTOW determines how many tires of what weight rating are needed.

A more important tire consideration is speed. The tires are rated at 204 kt. This paper gives an example of a 747 at 825,000 lb there is a weight margin of 30,000 lb. But liftoff speed is 199 kt, so there's only a 5 kt speed margin.

An aircraft will have a certified MTOW which it can't exceed, but keep in mind the actual MTOW for a specific flight can be lower (and often is). MTOW will always be the lowest of the following limits:
Field Length (runway length, condition)
Climb
Obstacle Clearance
Tire speed & brake energy
Landing Limit weight (i.e. can't plan to arrive at something greater than MLWD)

Another factor in MTOW is the ability to maintain control after takeoff. Just getting airborne isn't enough. Northwest 255 managed to lift off with flaps improperly set, but couldn't maintain control and crashed. Air Florida 90 also lifted off, but crashed almost immediately due to icing and less than full power from the engines.
An aircraft too heavy for the design can suffer the same fate. Ernest Gann described an incident where a C87 (B24 derivative) with an accidental overload of fuel he was flying out of India during WW2 almost hit the Taj Mahal, because it would barely stay airborne with no ability to climb or change direction without stalling. Only a last minute application of flaps got it over the towers.

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text:
Does this trip count for the commercial 300NM cross country requirement?
I'm an instrument-rated PPL and recently flew (M20J) solo from NJ to Florida with one leg of 280NM and 3+ landings (not counting the return flight because I had 2 passengers).
Will this trip satisfy the commercial cross country requirement of 300NM solo when I start my training next year even though I'm not currently training for my commercial?

Related: https://aviation.stackexchange.com/questions/88028/61-133b1-what-counts-as-a-full-cross-country-flight , https://aviation.stackexchange.com/q/55825/34686

Yes. It counts. One landing needs to be at least 250NM away and you have a 280NM leg.

§61.129   Aeronautical experience.
(4) Ten hours of solo flight time in a single engine airplane

(i) One cross-country flight of not less than 300 nautical miles total distance, with landings at a minimum of three points, one of which is a straight-line distance of at least 250 nautical miles from the original departure point. However, if this requirement is being met in Hawaii, the longest segment need only have a straight-line distance of at least 150 nautical miles;

It depends.
If you go part 61, then absolutely.
If you enroll in a part 141 course, then they will have the flight as part of their training course outline and therefore will require an additional completion in sequence with their course unless they have a way to waiver it upon request, but I think they can only credit entire ratings, not single flights.

Legally, yes. That being said, for a commercial certificate they like to see you plan and execute flights like that under VFR using pilotage and/or dead reckoning skills to navigate the route.  You also should remain very sharp on those skills even as an instrument rated pilot.


text:
Is Airbus offering to retrofit older A350s with the new winglets?
I noticed that new winglets (sharklets?) on the A350 are extended as part of the PIP (Performance Improvement Package).
Are older A350s being retrofitted with the new sharklets? Does Airbus even offer that as an option? I'm assuming it's probably not as simple as swapping out sharklets because we're talking about a composite wing here.

Composite wing doesn't mean that its one-piece. I don't know the answer to this question, but I'm sure the winglets can, at the very least, be removed/replaced.


text:
How can I tell if an aircraft is equipped with fly-by-wire or with a FADEC?
Can anyone help me with a link or advice on how I can identify whether an aircraft is fly-by-wire (FBW)?
I understood the function of FBW, however if I want to know if a SA227 or smaller type of aircraft has FBW, I am not sure how to identify it? Is there a link or a way to exactly say which type of aircraft have FWB and which don't?
The same applies for FADEC as well.

Generally you use a search engine to look up the airplane, and read the description of its control system.

Sa227 is not FBW.  It has a single redline computer (SRL) but no FADEC.

thank you for feedback

Only an aircraft model can be fly-by-wire or not, not an individual airframe of the same model.
Designing FAA/EASA certifiable fly-by-wire systems is extremely expensive, so only expensive aircraft, produced in large numbers, are going to have it. All Airbus since the A320 and Boeing 777 and 787 airliners are fly-by-wire, so are high-end Dassault bizjets. Recently the club has been joined by Bombardier, Embraer, Sukhoi, Irkut (Yakovlev) and Mitsubishi.
This doesn't include military aircraft, though you'll largely see the same players there with the addition of Lockheed, Northrop, MiG, BAE and a few other all-military contractors. Air forces have more need for fly-by-wire and are more risk-tolerant than airlines.
The SA227 is not fly-by wire. For the most part, if you have to ask, or if it's affordable below corporate level, it's probably not fly-by-wire.
EEC (of which FADEC is the most advanced subtype) versus hydromechanical engine control choice is specific to the engine and the airframe. It's not readily visible. An airframe will either have cables and hydraulic lines or sensors and wires. The majority of jet airliners currently flying use EEC or FADEC engines.

All currently in production Airbus aircraft are fly-by-wire, but all currently flying definitely not—there is still quite a few A300 and A310s still in service, mostly as freighters.

Therac /Jan Hudec: thank you for the feedback, very informative.

There are two ways to tell if an aircraft is fly-by-wire. The first is to open the access panels, trace the wires coming from the controls, and see if they lead to a computer or not. The other way is to look it up in the aircraft's documentation.
The same thing is true for FADEC, except that FADECs are specific to the engine, not the plane. It's entirely possible to replace a non-FADEC-equipped engine with a FADEC-equipped one and vice versa. So, if you're going to look up the answer, you first need to identify what engine it has.

It is not relevant whether wires coming from the controls lead to a computer. What matters is whether the control column is connected via pulleys, hydraulic lines, or electric wires. It is also the control column specifically—there is a computer even in a modern C172, the G1000, but it does not make it fly-by-wire.

@JanHudec That's a common myth in the aviation community, but it's not true. Fly-by-wire is a term that specifically indicates computer control of the control surfaces. Just because an airplane uses electric wires to actuate servos does not make it fly-by-wire. If you google "fly-by-wire", you'll see that this is confirmed by both general knowledge sources (Wikipedia, USA Today) and by aviation-specific sources (AOPA).

I don't think Wikipedia says it must involve computer specifically. It even includes analog systems in the examples…

@JanHudec Why, so it does. I didn't realize that. However, at the time if writing this, if you google "fly-by-wire", every link on the first page of results says that computer control is a necessary component of the definition of the term, except for Wikipedia and one link back to this very site. This includes a dictionary of aviation terms, the AOPA's website, and...

... two different magazine articles. I still think I'm right.

… notice how the skybary and aopa pages don't even agree on which was the first fly-by-wire aircraft, and, more importantly, the second even contradicting itself, because it starts with saying “computer”, but then it says the first was Avro Canada CF-105 Arrow where it is a really long stretch to call anything a “computer”. Not as if it was an official term anywhere anyway.

@JanHudec If there's no official definition, then why the downvote? Even if you discount the skybrary link, the vast majority of references still support computer control.

I don't know. I didn't vote either way.

@HiddenWindshield: just so I get it fully right. It means that most of jet engines are FADEC equipped but  turboprop engines on an aircraft can be upgraded to be FADEC equipped right ? thx

@JanHudec Oh, sorry. I apologize for assuming.

@BamboBamborowicz Well, the engine itself can't be upgraded, as there are almost insurmountable hurdles at both the legal and technical levels. But, the engine can be removed and replaced by a different engine entirely. Most of the big commercial jet engines are FADEC equipped, but its certainly not a requirement, and there are plenty of non-computerized jet engines out there.


text:
Does a decreasing shape factor correspond to greater or reduced probability of separation?
The shape factor ($H=\frac{\delta^*}{\theta}$) is the ratio of displacement thickness, a measure of the thickness of the boundary layer, to the momentum thickness, a measure of the shear stress. I think that a larger shape factor indicates that the boundary layer is larger and there is less shear stress, so it seems like a larger shape factor should be associated with a greater likelihood of separation. Is that right? If not, why?

There is no need to close this question. It is very clear what OP is asking.

@Koyovis It might be that I used the separation tag wrong.

Yes you're reasoning is spot on. As the momentum thickness decreases, that indicates the velocity gradient at the wall is less, indicating a greater likelihood of separation occurring.

If you answer your own question, more explanation and background is useful.


text:
What are the stability criteria of helicopters?
In airplanes, longitudinal stability is maintained if centre of lift lies aft of center of gravity.

Like in the airplane shown in the figure below, do helicopters also have stability criteria?
They have main and tail rotors. How is stability affected by their location?
Does the centre of the main rotor always correspond to the centre of lift?

I have read that airplanes are more stable than helicopters. A diagram showing point that affects stability on helicopters is much appreciated.

The diagram there is really describing trim forces, not static stability forces. In any case, the helicopter has neither.

In hover, helicopters are unstable and there is nothing that can be done about it. Being banked does not create any restoring moment as lift is still along the shaft, but center of drag is below center of thrust, causing destabilizing moment.
In forward flight, the advancing blade creates more lift (unless countered by cyclic), which causes the rotor pitch up (the blade reaches highest point about 90° later due to gyroscopic effect), creating stabilizing moment that overcomes the destabilizing moment of low center of drag above certain speed. However there is no aerodynamic way to trim for speed like in airplanes (there can of course be adjustable springs in the control system).


text:
Does FedEx use other airlines?
I have an International Priority parcel being shipped by FedEx from Zurich, Switzerland to Perth, Australia.
Currently the parcel is located in Singapore, I presume with FedEx at Changi Airport.  It is scheduled to be delivered by 1700 on 09 October (GMT+8).  The parcel (tracking #771679606750) is showing in tracking at Singapore since 0636 05 October (GMT+8).
From what I can see on FlightAware and Flightradar24 there are no scheduled FedEx Express flights from Changi to Australia, at least up to Wednesday 07 October as I can't see past that date.
This raises the question given the parcel is scheduled to be delivered by 09 October (GMT+8), if FedEx uses other airlines for parcels, or for International Priority parcels?
Note that Sydney is the usual point of entry for FedEx, and Perth is a day's flight from Sydney, and another day for distribution at Perth.

Bear in mind that FlightAware adn FR24 are hobby sites, not reliable sources for accurate flight information

Then I guess this FedEx promise of more flights to Australia is out the window...https://newsroom.fedex.com/newsroom/fedex-express-launches-sydney-singapore-flight-to-support-australian-business-growth/

There are regular flights between Singapore Changi and Sydney operated by FedEx Express. Here is a list of past flights from Singapore to Sydney (FDX9075):

(flightaware.com)
There are regular flights on Monday, Tuesday, Wednesday and Saturday. You can see that there was no flight on Monday, 05 October. This is likely because the aircraft coming from Sydney (FDX9077) diverted back to Sydney on Monday:

(flightaware.com)
I don't know the reason for the diversion, but as you can see, there is an MD11 enroute to Singapore right now. It is therefore likely that the same aircraft will depart Singapore back to Sydney later today as FDX9075, possibly carrying your parcel.
Update: Return flight is in the air right now:

(flightaware.com)

To answer your question about using other airlines:
Normally, FedEx does not use other airlines for international flights. They operate their own network of flights with their own airline FedEx Express. In these special times, it is quite possible that things have changed1, but I could not find anything official.
FedEx is however using a lot of other airlines for local transportation. They are called FedEx Feeder airlines. Wikipedia has list of them. They only operate regional aircraft (most of them ATRs).
1 Normally, a lot of air cargo goes on passenger aircraft (below the passenger deck). Due to the COVID pandemic, passenger travel is significantly reduced right now, which means more cargo needs to be transported on dedicated cargo aircraft than before.

It should also be noted that during the holiday season, FedEx does wet lease aircraft from other operators to handle the significant uptick in packages. This can consist of 747’s, MD-11’s, and 767’s. However, this is usually only for the month preceding Christmas, and sometimes for the month following.

@Bianfable  Great answer, thank you very much.  Actually the package is now in Welshpool, Western Australia.  This is where Perth Airport is located, and also FedEx. https://local.fedex.com/en-au/welshpool/pera/     I didn't think it would get here so fast after showing as in transit at Singapore....there is no tracking stamp for Sydney, and I didn't think FedEx flew directly to Perth...

I think bianfable answered what was going on in your specific case. Changi airport is the location of a large regional FedEx hub. To answer your general question, FedEx does sometimes place packages on pax airlines when something unusual has occurred, like it missed the flight.
I used to run missorts around the Memphis hub, usually to the aircraft itself. If I got there too late and the plane was closed up I would take the package to the expedite department. Depending on factors like, if it was perishable or a medical shipment, etc., and availability of pax flights they sometimes would send them with other airlines.
As far as regular movement of freight, the only outside airlines I know of is the feeder network that Bianfable mentioned. FedEx has a larger fleet than most pax airlines, so they move everything in-network.
When you look at FlightAware or FR24 the regularly scheduled flights are on there, but FedEx has a significant number of ad hoc flights that won't show on a schedule.

Looks like FedEx may have used another carrier for this parcel which is now with FedEx in Perth as from Wednesday 7th October.  The FedEx tracking shows no receipt for Sydney, just Singapore on Tuesday 6th October, then Welshpool in Perth, Western Australia on the 7th.
Scoot and Singapore Airlines both had direct lights from Singapore to Perth on Tuesday.  There were no FedEx departures listed on FR24 for Tuesday from Singapore.  Scoot and SA were the only flights to Australia from Singapore on Tuesday 6th.  I think it may have been Scoot, which is a budget flight subsidiary of SA  https://www.flightradar24.com/data/flights/tr8#25b1dbb9

Interesting. Maybe they put it on a different flight because of the diversion back to Sydney on Monday.


text:
What could cause the APU on a Boeing 777 to fail and also trip the GPU?
I am a cargo aircraft loader at Heathrow and while we were offloading cargo, pallets and baggage containers from a Pakistan Airlines B777, the aircraft's Auxiliary Power Unit (APU) failed. It made matters worse when we attached the Ground Power Unit (GPU) ground cables to the aircraft nose because the GPU went into trip mode. This resulted in a complete loss of power and we were unable to open up the front and rear cargo doors. The loading systems' Power Drive Unit (PDU) also failed.
My team leader played with the GPU leads and set them at a certain angle where the trip mode stopped and we offloaded the airplane. What could cause this APU failure, and why would the GPU trip? Is this a safety issue if airlines operate a non-functional APU on their aircraft?

A functional APU is not a MEL item for many aircraft/operators.

What is a Mel item sir.

@RAYHANahmed Minimum Equipment List -- it tells what you have to have (or how many) to operate the aircraft and still be complying with the regulations. If the MEL listed the APU as required, you could not operate without it. This would make a good regular question, in my opinion.

Dispatching an aircraft with an inoperative APU, regardless of whether it's a passenger aircraft or a freighter, is a normal procedure. While an APU may be started inflight preparatory to it's use after landing, it is not needed inflight, so it's not a safety issue. In my experience (retired in 1999), dispatch of freighters with inoperative APUs is more common than dispatching passenger aircraft that way due to the passenger comfort aspect. Keeping passengers cool while unloading/loading is a priority. Keeping a freighter's crew comfortable is not.
An APU failing while in use is not unusual. GPU failure is also not unusual. If the APU tripped (as opposed to just quit) and the GPU tripped as well, a natural suspect is an electrical problem in the airplane. What is needed, of course, is a competent mechanic to troubleshoot.
APUs can fail for a number of reasons. They're just small turbine engines. In fact, for 747-100 and -200 aircraft, the APUs were often the same as the engines used for SA-226 and -227 Metroliners (with the addition of propellers of course) that I flew prior to flying 747s. Common causes of APU failure include overheating, loss of oil pressure, excessive load, and just plain mechanical breakdown.
The decision as to whether to dispatch a large airplane without an APU would be an economic one primarily. If you're sending the airplane to an airport that is known to have GPU power, it wouldn't make financial sense to delay the flight because of an inoperative APU. If, however, you're dispatching to an airport that doesn't have such, that would be unwise unless such an airport does have a huffer and you want to risk a battery start (never a good idea in my opinion).

Thank you for your answer. I loaded freight pallets and cargo on many types of aircraft upto A380. One thing many airlines are doing coming in with non operative apu's this included major British airlines on there hub airports.

Huffers are used mainly on PIA Virgin emerates as noted.

Sidenote on APU inop dispatch -- ETOPS twins can't go ETOPS w/ an inop APU (I don't think it applies to three and four holers, though)

Sorry I am only a cargo aircraft loader. I DON'T KNOW

Sorry sir I am only a cargo aircraft loader.

@RAYHANahmed -- the comment was aimed at Terry, and other readers :) he's used to four-holers

Four-holers I get it you used play golf with him


text:
Why are movable rocket thrusters not used in airplanes?
If an engine fails in a passenger airplane, why can't it have movable rocket thrusters in the wings to help safely land the airplane?

because we have engines and wings for that.

@ratchetfreak and engines are thrusters.

And the fact that rocket fuel isn't something you want on board a plane... like, ever. Don't do rocket fuel, kids. Not even once

when emergency occur.when engine failure occur, or just have 2 engines?

"if an engine fails"..... so what happens with your rocket fails?

There is also the option of using the rocket to deploy a parachute -- many thousands of these systems have been sold. https://spinoff.nasa.gov/Spinoff2010/ps_3.html

@ratchetfreak  They aren't "moveable" though (unless you count the previously spinning blades of course)

We do sometimes have thrusters to help the plane take off though...   https://aviation.stackexchange.com/questions/42675/what-is-this-plane-and-what-are-the-rockets-on-the-back-used-for

Any time the question is "why don't we have...", the answer is almost certainly in the trade-off between weight, maintenance cost, fuel cost, & utility. If it weighs a lot, needs maintenance, burns fuel, and will only be useful once a decade, then it just is not productive.
Putting rocket thrusters on a plane will be heavy, meaning fewer passengers, or less fuel or cargo on board.   All that extra weight has to be flown around, requiring fuel.  They would need regular maintenance checks, which is another cost.   And they would only be useful very, very infrequently (on most planes... never).
So, safety issues completely aside*, it is just impossible to justify adding new equipment for a once-in-a-lifetime scenario, which can be better managed with good maintenance, good training, and good planning.
This same line of reasoning goes for "why don't more airplanes have parachutes", "why don't planes have anti-missile systems", "why don't planes have airbags", and many other random items.

*Safety issues are a big deal with rockets.  Rockets are a controlled explosion.  What happens if that explosion is not controlled?  What happens if the rockets are triggered by accident, or blow up all at once?  Even if rockets passed all the tests of utility vs. costs, they would still increase risks to the plane, rather than decreasing it.

"Movable rocket thrusters," regardless of propulsion, are used for thrust vectoring, AKA maneuvering, not forward motion.
If you're talking about something like an emergency JATO rocket, then the reason is because very, very few accidents happen strictly because of engine failure. Most engine failures (notwithstanding US Airways Flight 1549) happen at an altitude where the plane can pick a safe landing spot, and glide to the runway.

@abelenky  Thank you for solving my confusion in that part .

Note that the US Airways flight DID glide to a safe, if somewhat damp, landing.  At least for the passengers, if not the airplane.

"A good landing is one you can walk (or swim) away from.  A great landing is one that allows further use of the aircraft."

The Boeing 727 was certified with a rocket-assist system as an available option, in order to meet one-engine-inoperative climb gradient requirements for operation at extremely hot/high airports.  It was only ever selected for purchase by Mexicana airlines, and was delivered on 12 aircraft.  You can find videos of it being tested on Youtube.

The advantage of rocket thrusters over conventional aerodynamic control surfaces is that they work in space. This is not a factor for commercial airliners, but some research aircraft designed for very high altitude operation, such as the North American X15 have used fixed rocket thrusters in addition to the conventional movable wing and tail surfaces. The use of such a system in a commercial aircraft would in itself pose major safety issues, because of the highly reactive nature of the propellants used in these systems, such as concentrated hydrogen peroxide (HTP).


text:
Can the commercial night cross country be done under IFR?
The below doesn't mention anything about needing to be night IFR, so if you were to shoot an approach on this flight, would it be legal?  Also, if the instructor isn't a CFII, but PIC does have an instrument rating?
https://www.law.cornell.edu/cfr/text/14/61.129

(iv) One 2-hour cross country flight in a single engine airplane in
  nighttime conditions that consists of a total straight-line distance
  of more than 100 nautical miles from the original point of departure;
  and

You're in FAA-land I take it from the FAR reference? Also, I take it both pilots are IFR rated/current, just there's no CFII in the cockpit?

If the instructor is not a CFII then if the flight is in IMC the person is just going for the ride and not instructing. I don't know the answer to this question, but my initial reaction is to wonder why you would want to do your flight in night IMC?

I didn't, but where I fly marine layers some times come in even if not forecasted, so if the flight was vfr, then on the return the cloud deck came in, I'd rather shoot an approach through it than try to go under and scud run.  As it turned out no marine layer and no IMC required.

Think off the spirit of the rule, not so much the letter of the law.

In actual or simulated IMC you have removed the NIGHT part of the cross country to just  takeoff and landing. What about all those other nautical miles?

I don't follow how IMC removes the NIGHT part--It's still dark out.  And en route cruise seems like it'd be the same either way.  In my earlier example you fly to your destination and return.  You get the ATIS and it's fine and clear conditions, then as you approach the VMC turns to a 900 ft ceiling IMC situation, so you need to get a pop up clearance to shoot an approach.
Some times marine layers come in at 8pm, other times 2am.

The regulation doesn't specify VFR or IFR, so you may complete that requirement in either.

Just to corroborate this, I checked my logbook and I did my commercial night cross country under IFR. In my case, I wanted to stay instrument current and it was a good opportunity. It was VMC, but being under the hood at night is much closer to IMC than during the day.


text:
What older regulations and exemptions does Boeing's 737 line benefit from?
Boeing is occasionally accused in the press of keeping the 737 competitive by being able to avoid newer regulations and standards, either through being previously certified or by other means.
The oldest example I've seen is an exemption from FAA amendment on 2-second V1 margin for the 737-700 back in 1995, and the newest is Boeing's engine anti-ice exemption request for the MAX 7, which has since been dropped.
What differences between 1967 and 2024 regulations does Boeing's 737 line still benefit from, which wouldn't always apply to a new airplane of similar size and role?


text:
Why Is This Tyre Square On The BA A380?
One thing I saw on the news recently was the following image:

How has this happened?
Some details:

British Airways A380-800.
Flight BA-32 from Hong Kong to Heathrow.
While climbing out of Hong Kong crew received a tyre pressure notification, but continued on as the A380 can land with 1 of it's 18 tyres flat.
Landed safely, but had to request a tug to the gate.
The photo above is after landing.

I can't seem to find a clear explanation to why the tyre has gone this square shape. Generally when a tyre is flat it's round with the bottom being straight. The explanations seem to vary between tyre pressure, to the way it lands, to the way it is stored.
What's the real reason it's square?
http://avherald.com/h?article=498050d8

I am just staring at the tyre like "wut?". I have no words for that.

I think maybe we've found the answer to this question: What causes an airplane to rhythmically bounce during taxiing?

My guess would be the thermal relief plug (or whatever the A380 uses to relieve the pressure of a tire nearing it's thermal limit) deflated it, and then it spent 10+ hours in a wheel well that got rather cold eventually.

Looks like maybe it lost pressure at altitude and during descent the pressure caused it to collapse

Ad “continued on as the A380 can land with 1 of it's 18 tyres flat”: well, they were already in the air, so they had to land either way. The key think is whether it can fly with flat tyre, which it can as long as the gear retracts properly — a shredded tyre can sometimes prevent that and hot brakes may be a reason not to retract it.

A mythbusters episode?

@mins - so is asking why it went square (instead of the usual flat tyre shape) not suitable then? - Not complaining, jus asking

Reading again the question, I think I've been excessive, as the question is only about the shape. I apologize, and will revert my vote. You are right, it could have been a triangle or anything else rather than a square. Sorry again. The image is going to be famous!

I'm not sure I see how this is an aviation question. Just because the tire is on a plane doesn't make it necessarily so, unless the ONLY reason it is square is due to being on one....

Searching for "Tyre Vacuum" gives a few results that are very similar. Here's one from YouTube.

@CGCampbell - based on sweber's answer, I'd say that the only reason it's square is because it was on a flying plane, therefore, it's aviation related.

@FreeMan Agreed! Superb answer. And I have retracted my vote.

According to this document about the landing gear system, the wheel dimension  of the main lading gears is 1400x530 R23. This is not the standard notation known from car tires, but it says that the diameter is 1400mm and the width is 530mm, so the tread has a surface of about 2,3m².
The same document further states:

Depending on the flight phase, a warning will be triggered if a
  tyre pressure is detected below a given percentage of the nominal tyre pressure (for 5 seconds continuously).
  Likewise, a warning will be triggered if a differential pressure above a given percentage is detected across tyres on the same axle (measured from the tyre with the higher pressure).

though no concrete threshold is given.
The nominal pressure of the tires is 15Bar (atm) and will quickly fall, even when there's just a tiny leak. So no wonder there was a warning during climb.
During the 13h flight at up to 40.000ft, the pressure could easily fall to the environmental pressure of about 0.2Bar at that altitude. And from that altitude, it takes only 30min to landing (taken from flightaware for a BA32 flight), which may not be enough to equalize tire pressure to ambient pressure through a tiny leak. Also, ambient pressure will rise slowly in the beginning and reach 1Bar at ground quite late.
Considering the pressure in the tire was still 0.2Bar when landing, the pressure difference to the ambient caused a force of 176kN on the tread, which equals a weight of 18t or 39700lbs! (one could also say: 4.5t or 5000lbs on each of the sides of this square shaped tire.)
So it's no wonder that the tire was compressed, though this doesn't explain why the tire choose the square shape.
In general, the tire tries to minimize its volume while the surface is constant, but it can't change to any shape. Its stiffness and the rim only allow the tread to bulge inwards. From math, we know that the more sides a symmetric polygon has, the more surface it has w.r.t its circumference. So the less sides, the better.  A triangular shape seems not to be possible due to the rim and the needed large force to bend side walls. A pentagon doesn't provide enough reduction of volume, so a square is the best choice.
So though the picture looks really crazy, the square shape is not that surprising.
However, a tire does not think about how to collapse best to minimize volume. It starts an one or more points and evolves. If it starts with three bulges, it could have stayed with it, and the A380 had landed with a triangular tire. May be, a fourth bulge has developed at some point. Maybe, the shape in air was different, and some punched during landing pushed the tire into that shape.
Finally, one can only say that it's clear that the tire collapses to something like this due to the ambient pressure working against the low pressure inside, but one can not really predict the resulting shape. But the rectangular shape seems to be the best to minimize volume.

why the tire choose the square shape - I can almost see the thought bubble above the tire as it cruises along at 40000ft, shivering, thinking "I'm not under any pressure to make this decision, but I've got to pick something...". :) Excellent answer!

@FreeMan "not under any pressure" groan

@DavidRicherby I'm so glad somebody got that!!


text:
Are the aircraft used for the A380 display ready to be delivered to airlines?
For displays in international airshow, Airbus uses A380 whose livery is the one of customers. Here are some examples:

Le Bourget 2011: Korean Air
Farnborough 2012: Malaysian Airline
Le Bourget 2013: British Airways

Are those aircraft regular newly built aircraft ready to be delivered to airlines (with all the seats and passenger equipment inside)? Are those aircraft specially prepared?

@RedGrittyBrick I think F-WWAK is a temporary registration as it has been assigned to several A380 before being changed.

@PeterKämpf When asking the question, I was thinking of one aircraft used by Airbus (prototype, test,...) performing all the display flights and whose livery would be changed; or planes not yet ready to be delivered that need to perform flight test or whose interior is not yet complete

Airbus uses dummy engines when they paint the engine cowling just to avoid the capital cost of owning the engines for the few days the painting needs. All aircraft are delivered, and test or display articles are kept for the shortest time possible. A dedicated display aircraft is simply too expensive.

@PeterKämpf Yes a dedicated aircraft would be expensive as there is not airshow all year long, but there are prototypes still active (MSN001, MSN004) that could be used for such display.

The average price of an A-380 is 414.4 Mio USD. Just the financing cost at a yearly rate of 2% costs 22,700 USD per day. Such display aircraft need to be kept away from their customer for as little time as possible to keep airshow expenses down.

@PeterKämpf: Maybe they can put this one to use http://www.ainonline.com/aviation-news/business-aviation/2015-02-05/airbus-cancels-only-vvip-a380-order

In all cases the answer is yes, those aircraft were all outfitted with seats, and at least very close to being ready for delivery to the customer. Installing interiors and seats is a regular part of the production process, and it's unlikely that the plane would be pulled from the line before seats are installed, and worked back in later. From looking at rollout and delivery dates, these aircraft all had about 7 months between rollout and delivery, with the air show appearance happening within a month of delivery. Also, the aircraft are used as static displays while not flying, allowing people to walk through and see the interior.
In 2011, the Korean Air A380 indeed had seats in it. It was delivered in the middle of July.
In 2012, the Malaysian A380 also had seats installed. It was delivered at the end of July.
In 2013, the British Airways A380 probably had seats installed. I can't find any pictures of the inside, but it was delivered shortly after in early July.
Airbus also tends to bring a flight test A380 like MSN001 (first to fly, registered F-WWOW) for display, and has a more unusual interior. The Korean Air A380 was used for the display in 2011 after the flight test plane MSN004 had a bit of a mishap.

Are those aircraft regular newly built aircraft ready to be delivered to airlines

Yes. At least in the case of the aircraft in BA livery at the Paris airshow.
From other photos taken at that event, it had registration F-WWSK and construction number 095
BA registered it as G-XLEA.
That was listed as delivered 4 July  so almost certainly fully completed ready for BA when at the airshow on 17th June.

In the early days of A380 only prototypes were available, flying from airshow to airshow, each time with a livery of national company of the place.
They used giant adhesive films from Adhetec (13880 film series(pdf)).


text:
If I fly a balanced turn long enough, will the AI show I'm level?
Attitude Indicators are essentially gyroscopes. Mechanical gyroscopes have a "rigidity in space" property, which allows them to preserve their orientation when the support rotates. However, like spinning tops, gyroscopes will also slowly right themselves up due to gravity; otherwise the AI will show pitch down as the plane travels forward.
If I keep an airplane in a balanced turn long enough, is it possible that the gyroscope will right itself to the fake gravity, and show that I'm straight and level?
What about ring laser gyroscopes?

No, because in a turn the local “level” continuously changes.
Lets say you are heading north and turning right. So your floor is banked to the east and the gyro is accumulating bias to east too. But after you make 90°, you are heading to the east and your bank is now to the south, so the gyro is accumulating bias to the south while the bias to the east is already reducing. So the gyro will never show precisely correct level, but it will never indicate level in the turn.
Regarding whether ring laser gyroscope will behave the same, it is a bit more complicated. The righting of the gyroscope is not inherent property of the gyroscope. It is added to compensate for curvature and rotation of Earth. A gyroscope made not to right itself maintains the same axis of rotation relative to distant stars. But as you fly along the curved surface of Earth, and as the Earth rotates with you, different stars will be getting into zenith. So the gyroscope has to be continuously adjusted to the local level.
When you have single gyroscope for use as attitude indicator, it needs a righting mechanism whether it is a mechanical or ring laser one and that mechanism will cause the same kind (and similar magnitude) of error in turns.
However a complete inertial navigation system can improve the precision. Such system knows how you move and so it can calculate how the local level, relative to distant stars, differs from the initial and can include this correction. It will still do some righting, using average acceleration, to correct for drift, but it can take the average over longer time, so the error in turns grows more slowly.

A gyroscopic Attitude Indicator will not build up long-term error simply because of its erection mechanism in an extended coordinated turn. However, all gyroscopes will accumulate error over time, which must be corrected in some fashion.
A gyroscopic AI will display a turning error of 3-5 degrees in a 180 degree turn. However, this precession is reversed in the second 180 degrees of a turn, so after 360 degrees, the precession is eliminated and the AI is correct again.
To explain why this is, first consider the design of the attitude indicator: the pendulous vanes exert an erecting force when the gyro is not vertical with respect to downward acceleration.
(FAA AC 65-15A)
This causes the gyro to erect itself over time, and would in fact do so incorrectly if the aircraft were kept on a constant heading (if, for example, flown uncoordinated on a constant heading.)
However, in a coordinated turn, the case of the AI (including the pendulous vanes) rotates around the gyro. For the first half of the turn, the gyro is erecting in a particular direction. However, as the pendulous vanes move more than 180 degrees around the gyro, they add error opposite the original direction - removing the original error.

Note that while this fixes turning error in the long term, other sources of error are still in place. Precession caused by the fact that the gyroscope gimbals are not perfectly friction-free, which will vary in each individual instrument, will still happen - and the erection system will be unable to correct for it, as it's too busy causing and removing its own error.
A ring-laser gyro would be subject to the same problems, because it must still have a correction process of some sort. All gyroscopes, even perfect ones, are subject to apparent precession:

A freely spinning gyro tends to maintain its axis in a constant direction in space, a property known as rigidity in space or gyroscopic inertia. Thus, if the spin axis of a gyro were pointed toward a star, it would keep pointing at the star. Actually, the gyro does not move, but the earth moving
  beneath it gives it an apparent motion. This apparent motion is called apparent precession. The magnitude of apparent precession is dependent upon latitude. The horizontal component, drift, is equal to 15° per hour times the sine of the latitude, and the vertical component, topple, is equal to 15° per hour times the cosine of the latitude.

(FAA Flight Navigator Handbook, p. 3-7)
Any gyro, whether a ring-laser, MEMS, or gyroscopic, in a coordinated turn, will exhibit apparent precession and will have to be corrected over time if their only source of correction is acceleration.

Just found this page with great pictures of an AI from a spitfire. It's german, but you don't need to read the descriptions. However, I'm not yet convinced. The erection mechanism works slowly, it will correct the AI on a long flight, but not on a 360° turn. But does it really have no effect when flying a circle for hours?

Re "(if, for example, flown uncoordinated on a constant heading.)" -- seems to me that if the aircraft were flown uncoordinated on a constant heading, the erecting mechanism would align the gyro with the apparent "down" direction, which is the same as the true "down" direction.  In other words a pendulum points toward the center of the earth, not toward the cockpit floor, any time that the aircraft heading is constant.  Including in a constant-banked non-turning sideslip, or for that matter when the aircraft is started up on tilted on ground.  Which is exactly what we want to happen.

So the answer could be improved by modifying that one sentence.


text:
How do pilots manage metric altitude?
A minority of the world, specifically Russia and China, uses the metric system in aviation. ATC will tell the pilots to "climb to 900 meters height".
How do pilots of international flights manage this? Do they request the ATC to provide altitude in feet, or do they perform the conversion manually and dial in the altitude in feet to the autopilot?

In areas where the ATC provides the altitude in meter (China (PRC), North Korea, and Russia), depending on the equipment (altimeter), the pilot can determine what to do:

In case of older aircraft, where the altimeter is calibrated in feet, the pilots use a conversion card for determining the altitude in feet, before dialing in the altitude to the autopilot.

Source: aerosavvy.com
Note: The conversion charts are standardized for the airspace and the pilots have to use it. For Chinese airspace refer FAA document on Chinese RVSM

In case of modern aircraft displays, the altimeter can be made to display altitude in imperial (feet) or metric (meter) units.

In the latter case, it is critical to remember which way it is set!

Here is a page with all the info on this subject: http://aerosavvy.com/metric-imperial/

Altimeter Calibration: mb, hPa & inHg
  Altimeters can be set using hPa or inHg.
Altimeters can be set using hPa (mb) or inHg.
In order to calibrate our altimeters for varying atmospheric pressure, we set the current pressure reported by ground stations. Pressure is reported in inches of mercury (inHg), hectopascals (hPa), or millibars (mb). North America and Japan use the inches of mercury measurement. Hectopascals dominate the rest of the aviation world (millibars and hectopascals are equal). For years, altimeter manufactures have provided two calibration windows for hPa and inHg. This makes setting our altimeters easy no matter where we fly.  Just dial the reported pressure into one of the windows and you’re all set! Newer, digital altimeters have a button to switch between units. I like the older ones, they’re easier to set.
  Measuring Altitude: Feet vs. Meters
Here’s where things get tricky. Because of the proliferation of American and British aircraft during the early years of aviation, the imperial foot became standard for altitude measurement. China (PRC), North Korea, and Russia, however, use meters for altitude measurement.
As luck would have it, I often fly over Chinese, Russian, and North Korean airspace. The altimeters in our older aircraft, like the 767, are calibrated in feet. When flying into metric airspace, we use a conversion card. When Shanghai Control clears us to descend to 3600 meters, we check the card and descend to the equivalent: 11,800 feet. Does this sound like a pain in the you-know-where? It is!
During rush hour at airports like Shanghai and Beijing, the controllers rattle off clearances very fast. They will often assign us a heading, speed and metric altitude all in one shot. Doing the metric altitude conversion adds an additional task to an already challenging environment. We stay on our toes in metric airspace! The good news is that newer generation airplanes have a switch that can change the aircraft’s native altimetry display from imperial to metric units. When crossing into metric airspace, the crew hits the switch and the airplane understands the metric system. This reduces crew workload considerably.
  Distance Across The Ground
World-wide, the nautical mile (nm) is the standard for measuring the distance an aircraft travels across the ground. That’s nice and easy and it makes me happy. :)
Other lateral measurements are a mess. Most of the world measures runway length in meters while North America uses feet. Most of the world measures airport visibility in meters. North America? Not nautical miles, not meters, but statute miles! Huh?? Not to worry, North America changes back to feet when measuring Runway Visual Range (runway visibility measured with a laser), while the rest of the world sticks with meters. Confused? I sure am, and I do this for a living!
  Wind speed: Knots vs. Meters/Second
When reporting weather, airports in China and Russia state the surface winds in meters per second (m/s). They love this metric stuff! The rest of the world reports wind in knots (nautical miles / hour). Converting is easy:
1 m/s ≈ 2 kts
Double the m/s and we are close to knots. If Shanghai tower tells me the winds are 7 meters per second, I know they are blowing just a little shy of 14 knots. I can land in that much wind. Any more than that and I’ll let my first officer do it!
  Is flying safe with all these different units?
Yes!  Pilots flying international routes deal with this assortment of units daily. We can juggle them in our sleep. While it’s certainly an added challenge, it’s an important part of our occupation. There are very well defined standards and procedures that pilots use to work with the different units. Flying continues to be the safest form of transportation available.

Basically, it is that either the pilots have tables to convert units or the instruments can show both types. In either case, it would be very easy to follow the ATC instructions.

You may sum up the quote. Yes it answer the question extensivly, but it also add a lot of additionnal informations. You should put only the relevant information in your answer and provide the link for further readings. It would ease the reading of your answer

Now you've just copied and pasted the whole thing with no attribution and no effort on your part.  The OP can use Google for that.  This is no how SE.Aviation works.


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How can an airplane land if a hail storm damages the windshield?
I understand hail can be very damaging to an aircraft, in particular to the engines and windshield.
In the event that a windshield becomes cracked so that visibility is impossible, can pilots pop out the windows when at a lower enough altitude in order to see the runway?

Similar: What would happen to a flying aeroplane if there's suddenly a brick-size hailstorm?

Yes but doesn't address landing visually if unable to see through wind shield ☺

Related: How can landing in zero visibility be safe?

Not an answer, but looking at recorded incidents, it seems very rare that both pilot and co-pilot windshields become so damaged that neither pilot can see.

First, you might specify what kind of airplane you're asking about.  I for one find it rather disconcerting when questions unexpectedly turn out to be about commercial airlines, when my answer applies to SEL.  Where what you do if the view through the windshield is obscured is to pop open the side vent and look through that.  In my fairly limited experience, in a light airplane this happens mostly when landing to the west near sunset.

Quite remarkable this older question pops up today with two new answers, not mentioning the present incidence of delta airlines 1889: http://flightclub.jalopnik.com/flight-from-hell-hail-pummels-delta-flight-in-mid-air-1722890736 .

@sweber I'd assume that was the reason it popped up today. There was also another question asked related to that incident. Interestingly, only one of the answers even mentions who this actually works in large airliners: they just use auto-land (which is what happened in that Delta flight.)

In the EMB-145 the abnormal procedure for obscured windscreens is to secure items in the cockpit, open the side window and look through its opening.
There are more aircraft that share this procedure.
Here is the QRH procedure for "Impaired or cracked windshield" in the EMB-145:
Associated Windshield Heating .............................. OFF

IF  only the outer layer (glass) is cracked, no action is
    required

IF  inner layer of glass is cracked, proceed as follows:
    Cockpit Door ......................................... CLOSE
    Maximum Altitude ....................... 10,000 FT., OR MEA,
                                             WHICHEVER IS HIGHER
    Pressurization Manual Controller ........ 1 O'CLOCK POSITION

                    Wait 15 Seconds

    Pressurization Mode Selector ........................... MAN
    Cabin Delta-P ........................... SET EQUAL TO 1 PSI

    Note:  Pressurization Manual Controller must be used to set
           and maintain Cabin Delta-P at 1 psi while descending

    Airspeed .................................... BELOW 250 KIAS
    Smoke Goggles .......................................... DON
    In case both windshields are impaired:
    Cabin (below 10,000 ft) ....................... DEPRESSURIZE
    Airspeed ................................. MAXIMUM 140 KIAS,
                                                    MINIMUM Vref

IF  forward view through both wind screens is impossible, secure
    loose objects in cockpit and proceed:

    Note:  Intercommunication will be impossible with window
           removed.

    Direct Vision Window ................................ REMOVE
    Landing must be made looking through the direct vision
    window

                              * * * *

The pilot is also required to hang his or her tongue out and pant (or maybe that's just the MD-80?)

Opening a side windows does make more sense a bit less windy ...

@PaulYoung: The windows are diagonal, not completely parallel, so they get quite a bit of ram air, but you don't have to stick head out to look forward. They are the only openable windows in an aircraft too.

They use instruments instead.
BA flight 9 had it's windscreen rendered somewhat opaque by volcanic ash

As Flight 9 approached Jakarta, the crew found it difficult to see anything through the windscreen, and made the approach almost entirely on instruments, despite reports of good visibility. The crew decided to fly the Instrument Landing System (ILS); however, the vertical guidance system was inoperative, so they were forced to fly with only the lateral guidance as the first officer monitored the airport's Distance Measuring Equipment (DME). He then called out how high they should be at each DME step along the final approach to the runway, creating a virtual glide slope for them to follow. It was, in Moody's words, "a bit like negotiating one's way up a badger's arse." Although the runway lights could be made out through a small strip of the windscreen, the landing lights on the aircraft seemed to be inoperable. After landing, the flight crew found it impossible to taxi, due to glare from apron floodlights which made the already sandblasted windscreen opaque.

From Wikipedia

Some airplanes, like the Pilatus PC-12 pictured below, have a captain's side window that can be opened during landing so that the pilot has a direct view of the runway. If the windscreen is damaged, blocked, or iced-over, the captain can fly on instruments to the airport, then land by opening and looking out the little window.

If the aircraft has what is known as auto land, the auto pilot can land the airplane, so the pilot has that as an option. However, you can approach the runway in what is known as a forward slip. Using a lot of rudder, the tail will move to one side, this moves the nose to the opposite side, allowing the pilot to see forward, looking out the side window. The pilot also simultaneously uses opposite aileron, to counteract the turn, using the rudder would initiate. Once over the runway, at about 50 feet, the pilot would straighten the plane out, and begin the landing flare. He can then use the view of the the runway edge line, out his side window, to keep the plane aligned with the runway. A radar altimeter gives precise reading of how many feet he has to go, as he descends to the pavement. So, the pilot knows where he is. These things are practiced by flight crews in simulators twice a year.

Pilots tend to use the view out of the side windows in their peripheral vision to judge height above the runway, so provided you could see the approach lights at some point to know you are on the centre line, its not as bad as it sounds.
There are numerous examples in history of aircraft landing with oiled up, or iced up windscreens. Also it might be possible in some situations/aircraft to fly a curved approach then use the side windows in the final seconds of flight.
My view is that the reason why the wind shield is opaque was probably a bigger risk to the aircraft than actually landing with it in that condition.

Commercial aircraft pilots are required to pass an "Instrument Flight Rules" (IFR) rating.  This rating requires the pilot to flight their aircraft using just the instruments in the cockpit.
As to how will the pilot know where to land.  Commercial and military aircraft possess multiple instruments to provide location information.  This includes GPS based instruments.  I believe that this still includes VOR/DME (VHF omnidirectional range/distance measurement equipment) too but I've been out of the industry for a couple of decades and these may no longer be included.
At commercial & military aircraft flight speeds, busting out your windscreen would be a terribly bad idea - probably fatal.  It would certainly make it so you wouldn't be able to land the plane.  A better choice for non-instrument rated pilot would be to have the tower talk you in.
Per this other SE question & answer, although a true zero-zero landing is actually possible and safe to perform (provided ILS equipment is installed in aircraft and airport), there are problems on the ground with rollout and maneuvering on the ground which for practical purposes eliminate it except under extreme circumstances.

That is not technically true:  A commercial pilot can be VFR-only.   Although, admittedly, it is very rare, and not a very useful certificate to hold.

An IFR rating alone does not include training for zero-zero landings.

An instrument rating will only get you down to 2400 or 1800 ft RVR and 200 ft ceilings. To go lower requires additional training that must be done annually.


text:
In what conditions is the APU used in midair?
In flight, engines provide all the needed energy (bleed air, hydraulic and electrical power). So the APU can be shut down shortly after engines are running. But I imagine that starting the APU in midair is a well known procedure because:

I understand that starting the APU while the engines are still running is common given the answers to this question (even if done when the aircraft has landed)
In the US Airways 1549, the APU was started in midair

How is the APU used in midair? When is it started/shut down while the aircraft is not on the ground?
If needed, the answers can be restricted to airliners.

This answer applies to the AVRO RJ (BAe 146), because this is the aircraft I've flown. Other aircraft may or may not have similar features/issues/restrictions.
Reasons for APU use in mid-air

For takeoff, the APU bleed-air can be used to supply the cabin air pressure system to reduce load on engines. This leads to a significant reduction in runway length required. This is routinely used on short runways.
Similar reason for the landing, where, for the missed approach/go-around phase, a high climb gradient is required. For example in mountainous areas or combined with icing conditions, where use of engine bleed air is limited, the APU bleed air can feed the cabin pressurization.
During flight, when in icing conditions. Similar reason as above. The air-frame anti icing system uses that much bleed-air, there is not much left for cabin pressurization (or the other way round). If the APU bleed-air is used to pressurize the cabin, the (already low) climb gradient is improved.
During flight, in the event of a generator failure. The generator of the APU can replace one failed generator of an engine.
To save engine life. If APU bleed air (- to pressurize cabin) is used for normal takeoff, the peak engine temperatures will be lower during takeoff. This increases engine life as the peak engine temperature is the main factor of engine life. It's significantly cheaper to replace an APU than an engine.

In all of the above cases, there are restrictions on the operation of the APU. It may not light-up above 15'000 ft or operation above 25'000 ft is not allowed or not recommended. General operating rules for the APU (common sense when using turbines) recommend to use bleed air only after couple of minutes warm-up after startup. Likewise, on shutdown, it should be left running without load to cool down for a few minutes before switching off. The exact timings differ greatly between APU models.
Procedures

Before Takeoff, If the APU is not used during climb, the APU is normally shut down during taxi checks. If the pilots decide to use the APU during climb, it is started during normal engine start. In this case the APU bleed air is used to feed the cabin pressurization.
After Takeoff depending on the model of the APU (and following airline procedures), the APU usage is limited to a specified time or altitude. Before reaching this limit, the APU has to be shut down. (cabin air changeover to the engine bleed air, cool-off period of the APU, switching off)
During Flight In case of a technical problem or emergency, the Abnormal Checklist will suggest or dictate the use of the APU for the remainder of the flight. For example: A generator problem, the APU generator replace the failed generator. This may impose limits to flight altitude and duration.
Before Landing Usually after passing 10'000 feet, the APU is started and before the final checks, the cabin pressurization system is switched to the APU.

Auxiliary Power Units (APUs) are small gas turbine engines used primarily during aircraft ground operation to provide electric power, air conditioning etc and to start the main engines.
In flight, the APUs can be used for the following:

The APUs can be used to provide backup electric power during in-flight operation, in case the main supply fails (or is unable to meet the demand)
They can also be used to reduce load on the main engines when power requirements are high (for e.g. in hot and high conditions).
In case of an emergency in flight, they can be used to start the engines (Note: The engines can be started in mid air using either cross bleed air or windmilling)

Another thing is that even if the APU fails, most aircraft can operate without any issue using the Ram Air Turbines (RAT).

On the A320, when the RAT is deployed, and the engine generators are inop, only essential electrical buses are operative, see 4th row (emergency generator) in this table from the FCOM. Similarly, only the blue hydraulic circuit is pressurized.

RAT (on A320) has just 5kVA. Compared to engine and APU generators that have 90kVA each it is clear it can't power everything (or much at all).

If there is a generator failure in one engine (or an engine problem), the APU can be started to provide electrical power if total demand exceeds the capacity of the remaining engines generators.


text:
Can airliners operate without an operating APU?
As the APU (Auxiliary Power Unit) can be turned off when the engines are running, a functional APU may not be needed given you are able to start at least one engine. This question is in two parts:

Is it technically feasible to operate an airliner without APU (with the necessary ground support)?
If so, in what conditions is it legal (number of passengers transported, MEL, test flight,...)?

This question focuses on airliners. If needed, it can be narrowed to best-sellers (B737 and A320).

Lack of a working APU was a factor in the crash of Air Ontario flight 1363. The airport didn't have a huffer so they couldn't shut down the engines. Airline policy wouldn't let them de-ice with an engine running.

What's APU? Some sort of power unit I guess?

@Alec Teal APU = Auxiliary Power Unit

Inflight-startable APU is a requirement for ETOPS, so that's one condition when it's not legal to have none.

@AlecTeal You may find a brief definition and related question under the approprirate tag description

Yes, you can operate without an APU.  If the APU is inoperative you will need to write it up and coordinate with maintenance and dispatch to operate under the stipulations of the MEL.  This may restrict certain kind of operations but if we assume a route completely over land this has no effect other than needed ground services.
The requirements for no-APU is simply a ground start "huffer" cart at your departure airport.  Ideally you want one at the destination as well but if not available you can possibly coordinate leaving an engine running during the turn.  More generally though, if the destination does not have access to a huffer cart you will get an aircraft swap and the non-APU aircraft will be assigned to a route that does have those ground services available.
The general process of a non-APU flight is keeping ground power and ground air conditioning hooked up to the airplane for pre-flight activity and boarding.  Once the airplane is closed up the ground crew will coordinate with you to provide air via the ground start cart and you'll start one or both of your engines.  After you have an engine running, you'll disconnect all of the ground equipment and push back.  During the taxi, if you only started one engine earlier you'll perform a crossbleed start to start the other engine.  The takeoff engine bleed and pack configuration may be different for a non-APU takeoff but once you are airborne and set the bleeds and packs for post-takeoff you will be operating exactly the same way you would normally operate and the differences cease.
On arrival the only differences are to make sure you keep an engine running at the gate until ground power is hooked up.
The biggest downside to operating without an APU is the lack of decent air conditioning during taxi and ground operations.

Other considerations for operating without an APU are airplane specific.  For example, the EMB-145 has two generators per engine and one on the APU for a total of five generators.  The airplane will revert to "essential" power if less than 3 generators are available which means you will lose certain displays and systems that become unpowered.  This means if you had an engine failure during flight without an APU you will be in essential power and have to land in that condition because you cannot start the APU to get back up to 3 generators operating.

There is a story from Terry, in answer to completely unrelated question, that mentions both keeping an engine running and how hard getting a start cart can be in some 3rd world places.

@JanHudec sometimes it doesn't even have to be third world.  I nearly got stranded in a big city in FL because our ops' cart was broken but they told us they could borrow one.  When it was time to push we found out theirs was broken too.  It took quite a while but they finally got their hands on an ancient one that barely started and after two hung starts and dry motoring we finally got one of the engines started.  Fun times for all.

Yes it is.
Without the APU the airconditioning and electronics also won't work when the engines are shut down, so a ground power unit is connected to provide the electricity and enough compressed air to run the air-conditioning.
Starting the engines will need a air start unit to start one of the engines using compressed air provided by hoses. After that the bleed air from the first engine is enough to start the second.

I only like to add: depending on model an make of the aircraft, there may be battery-start available for the engines, including a limited time of ground-only battery operation.

If it isn't on the MEL, then it is required to be operational.

Is it technically feasible ?

Yes. Most aircraft switch off their APUs in flight. Also, APU is not mandatory for ground operations if external (Ground) power unit is available. Engine start can be done with an external unit or (in some cases) battery.

In what conditions is it legal?

That depends on a number of factors. In most airline operations, APU is not mandatory. In ETOPS operations, FAA requires that the APU be present if it is required for the aircraft for complying with ETOPS requirements. Also, APU should have in-flight starting capability, whether it is required for operations or not as long as APU is required by aircraft type certification.
In some extreme (for e.g., hot and high) conditions, the airliners should have a functional and operational APU as required by the regulatory authorities.

For non-ETOPS the APU is not required if engine generators operate normally. Here's the excerpt from the 767 MMEL.

If the carrier doesn't have a spare APU, it can be removed for repairs and the airplane operated if the MEL requirements are in-place and appropriate weight and balance changes are made.